The present invention relates to a process for the separation of nitrogen from a gaseous mixture containing nitrogen and oxygen.
Processes employing zeolitic molecular sieves for separating nitrogen/oxygen mixtures are known from, for example, US Patent No. 3 564 816, British Patent No. 1 437 600 and an article entitled "Molecular Sieves" in Soc of Chem. Ind. (London), 1968, by D. Domine and L. Hay at pages 204-216. The processes described all utilise zeolites in which the silica to alumina molar ratio is less than 10, such as zeolite 5A, zeolite 13X, mordenite, chabazite and erionite.
It has now been found that an MFI type zeolite gives an improved separation as compared with the above described zeolites. The term 'MFI-type Zeolite' is defined in the Atlas of Zeolite Structure Types by W.M. Meier and D.H. Olson, published by the Structure Commission of the International Zeolite Association, 1978. MFI type zeolites are different not only in structure but also in silica to alumina molar ratio from the zeolites which have been previously described for this separation. An MFI zeolite always has a silica to alumina ratio of at least 10.
According to one aspect the present invention provides a process for the separation of nitrogen from a gaseous mixture containing nitrogen and oxygen which process comprises contacting the gaseous mixture with a bed of solid adsorbent in an adsorption stage to selectively adsorb nitrogen and thereafter recovering a gas enriched in nitrogen from the adsorbent in a desorption stage characterised in that the adsorbent is an MFI-type zeolite.
During the adsorption stage an effluent from the bed enriched in oxygen is produced and this can be recovered, if desired.
According to another aspect of the present invention a process for the separation of nitrogen and oxygen from air comprises contacting the air with a bed of zeolite in an adsorption stage to selectively adsorb the nitrogen and produce an effluent enriched in oxygen characterised in that the zeolite is an MFI type zeolite.
The MFI-type zeolite used in the present invention can be that described and claimed in US Patent No. 3 702 886, although in this patent the zeolite is referred to as ZSM-5. Thus the terms MFI and ZSM-5 are synonomous. Generally, MFI-type zeolites are obtained by hydrothermal crystallisation from an aqueous gel containing a source of silica, a source of alumina and a mineralising agent or inorganic base. In the process of the aforesaid US Patent No. 3 702 886 the aqueous gel also contains a quaternary ammonium compound, generally regarded as an organic template. MFI-zeolites produced according to this method are not preferred for use in the process of the present invention because of problems associated with the remowal of the quaternary ammonium compound from the zeolite in which it tends to block the pores and thereby impair the zeolite's adsorption capabilities. This is true also, but to a very much smaller extent, of zeolites prepared using alkanolamines as nitrogenous organic bases, such as for example those described in our European Patents Nos. 2899B and 2900B. The preferred MFI-type zeolites are those crystallised from a gel containing a source of ammonium ions, as described in our European Patent No. 30811 which is incorporated herein by reference, or from a gel containing only a source of alumina, a source of silica and an inorganic base, as described in US Patent No. 4 257 885 or as described in US Patent No. 4 452 907.
As prepared, MFI-type zeolites may contain cations other than those desired in connection with their use as adsorbents, which cations may arise for example from the mineralising agent and/or inorganic base and/or the nitrogen-containing base used in the preparation of the material. The zeolite may also contain residual nitrogen-containing base lodged in the pores and/or deposited on the surface thereof and also molecules of the liquid medium employed in its preparation. Before use as an adsorbent, it is desirable that residual organic base and liquid medium be removed by calcining the zeolite, suitably at a temperature in the range from 300 to 7000 C. Preferably the zeolite is in the sodium or lithium ion exchanged form. Other suitable cations are those of Group IIA of the Periodic Table of the Elements as found in Advanced Inorganic Chemistry, Interscience, 2nd Edition by Cotton and Wilkinson. Techniques for cation-exchanging zeolites and separating the cation-exchanged zeolite from the exchange media are now well established in the art. The process of the invention may suitably be operated as a pressure swing adsorption process in the manner described for example in US Patent No. 3 564 816 in which the desorption is effected by pressure reduction. Alternatively, the process may be operated as a temperature swing adsorption process.
Conveniently the pressure during the adsorption stage is from 1 to 10 bar absolute preferably in the range from 1 to 4 bar absolute.
Conveniently the pressure during the desorption stage is from 0.1 millibar to 1 bar absolute preferably in the range 0.1 to 1 bar absolute.
Preferably the duration of the adsorption stage is from 20 seconds to 15 minutes and the duration of the desorption stage is also from 20 seconds to 15 minutes. The process may suitably be operated at ambient temperature, though lower and higher temperatures may be employed.
Conveniently the temperature in the bed is within the range -50 to -100°C and the process is conveniently operated isothermally without heat being supplied or extracted.
Conveniently a purge stage is interposed between the adsorption and desorption stages in which the pressure is reduced to a level intermediate that of the adsorption and desorption stages.
The effluent from the purge stage can be recycled to the feed to the adsorption stage.
The process of the present invention can be operated to produce an oxygen enriched gas (as effluent from the adsorption stage) of at least 30 % vol content oxygen, preferably at least 45 % vol.
The zeolite may be used with or without a binder and may be mixed, if desired, with other adsorbents.
The gaseous mixture comprising nitrogen and oxygen is preferably air, though other mixtures comprising nitrogen and oxygen in proportions other than those prevailing in air, may be separated by the process of the invention. The oxygen separated may be used in a variety of applications. The recovered nitrogen may be used, for example, in enhanced oil recovery operations. It may also be used for flare purging, process purging and a variety of other applications.
The invention will now be further illustrated by reference to the following Examples.
Sodium MFI prepared using ammonia solution and no organic base as described in US Patent No. 4 452 907 and thereafter converted by cation-exchange to the sodium form in the following manner:- the MFI zeolite was refluxed three times in about 1.5 of 1 M NaN03 solution for about 3 hours, washed with distilled water and then dried at 85°C after each reflux. The resulting sodium MFI zeolite had a Si : AI atomic ratio of 15.0, an AI : Na atomic ratio of 1.0 and a weight percent Na of 2.1. The zeolite was pressed at 4 tons in a 35 mm diameter die, broken up and sieved to 10 - 22 mesh (B.S.S.). The zeolite was not diluted or mixed with a binder.
The zeolite (about 4 - 5 ml) was outgassed at elevated temperature (400° C) to remove molecular water. After cooling, it was equilibrated with helium. A known volume (64.5 ml) of the oxygen/nitrogen feed gas mixture (room air) was passed over the zeolite in a single pass at room temperature (23°C) and one bar pressure absolute and the product gas, which was enriched in oxygen, collected. From the product and feed gas compositions (determined gas chromatographically) and their volumes, the amount of gas adsorbed, and its composition, was determined.
The selectivity towards nitrogen adsorption is given by the Separation Factor as follows:
- Separation Factor (N2/02) = vol.N2 adsorbed/vol.O2 adsorbed x vol.O2 fed/vol.N2fed.
- Details of the experimental conditions and the results are given in Table 1.
The procedure of Example 1 was repeated except that the sodium MFI zeolite was pressed to 7 tons instead of 4 tons and the volume of air passed was 65.5 ml.
The procedure of Example 1 was repeated except that instead of the sodium MFI zeolite there was used a lithium MFI zeolite, obtained by refluxing the sodium MFI zeolite four times in about 250 ml of 0.25 M LiNO3 . 3H20 solution for about 2 to 3 hours, washing with distilled water and then drying at 85° C after each reflux. Finally, the sample was pressed to 4 tons in a 35 mm diameter die, broken up and sieved to 10/22 mesh (B.S.S.) grade.
The procedure of Example 1 was repeated except that instead of the sodium MFI zeolite there was used calcium A-type zeolite (Molecular Sieve Type 5A supplied by BDH and manufactured by Union Carbide Corporation; Atomic Si/AI = 1.1; AI/(Na + 2Ca) = 1.2; wt.-% Na = 5.1; wt.-% Ca = 4.3) pressed to 4 tons in a 35 mm diameter die, broken up and sieved to 10/22 mesh (B.S.S.) grade.
The volume of air passed with 74.0 ml.
Details of the experimental conditions and the results of Examples 1, 2 and 3 and the Comparative Test are given in Table 1.
The oxygen content of the effluent from the adsorption stage was in each case 50.5 % and 50.5 % in Examples 1 and 2 respectively, and in the comparison the figure was 36.6 % by volume. The nitrogen contents were 48.4 %, 48.5 % and 62.4 % respectively.
The above results show that, (i) the MFI zeolites according to the present invention have a greater adsorption capacity for nitrogen than does the calcium A type zeolite which is a typical representative of the prior art, and (ii) the separation factor for the MFI zeolite is over 25 better than that for the calcium A type zeolite.
The results also show that the lithium form of the MFI zeolite has a higher adsorption capacity for nitrogen than the sodium form.
A sodium form MFI zeolite (Atomic Si/AI 15.3) prepared as described in Example 1 was used in repetitive adsorption-desorption cycles. 3.0 ml of zeolite granules, pressed at 4 tons in a 35 mm diameter die then broken up and sieved to 8 to 16 mesh (BSS), were placed in the adsorption tube and outgassed (ca. 0.1 mbar) at elevated temperature (ca. 370°C). After cooling to ambient temperature, dry air was admitted to the zeolite from graduated glass syringes. The adsorption was carried out by passing the air over the zeolite repeatedly until equilibrium was achieved. The adsorption conditions were as follows; duration 2 to 5 minutes, pressure 1 atmosphere absolute starting from the outgassed state of 0.1 millibar absolute.
The zeolite was then desorbed at a pressure of 0.1 millibar at ambient temperature (about 25°C) for 10 minutes and the adsorption stage repeated with dry air. Three cycles were made and the results given in Tables 2 and 3 below.
The dead space volume of the apparatus plus zeolite was determined by helium adsorption. Thus the amount of air adsorbed by the zeolite was determined. Gas chromatographic analysis of product and starting gases enabled the calculation of the adsorbed phase composition. Separation Factors were calculated as before for each cycle.
This demonstrates that all the adsorbed phase is readily recoverable.
For comparison the same procedure was used for 3.0 ml of the calcium A type zeolite granules (Molecular Sieve Type 5A supplied by BDH and manufactured by Union Carbide; Atomic Si/AI 1.1 pressed at 4 tons in a 35 mm diameter die, broken up and sieved to 8 to 16 mesh (BSS). The results for three consecutive dry air adsorptions are given in the Table below.
The above results show that the MFI zeolite can be used in a process involving repeated cycles of adsorption and desorption.
The results also show that the MFI zeolite has a higher separation factor than the calcium A type zeolite. The difference which is not as marked as that recorded in Table 1 is accounted for by the different process conditions.